Process of electrodepositing a corrosion resistant nickel-chromium coating



United States Patent 3 268,423 PROCESS 0F ELEtITRdDEPGSlTiNG A CORROSIGN RESESTANT NKCKEL-CHRGMEUM COATING Thaddeus W. Tomaszewski, Dearborn, and Henry Brown,

Huntington Woods, Mich, assignors, by mesne assignments, to The Udyiite Corporation, Warren, Mich, a corporation of Delaware N0 Drawing. Fiied Mar. 1, 1963, Ser. No. 262,200 7 (Ilairns. (Cl. 204-40) This invention is for improvements in or relating to decorative nickel electroplating, and more particularly relates to (l) the electrodeposition of sub-microscopic satin-textured to microscopic satin-textured fine-grained nickel plate from semi-bright and bright nickel plating baths containing dispersed therein fine bath-insoluble particles, and (2) to the exceptional corrosion resistance of these textured deposits when over-laid with a thin chromium plate.

The decorative fine-grained nickel deposits of this in vention have various degrees of brightness, or of uniform smokiness, depending mainly on the concentration and particle size of the dispersed fine powders in the semibrig-ht or bright nickel electroplating baths, the concentration of the nickel brighteners, the degree of agitation of the cathode or the solution, the brightness and smoothness of the metal surface plated upon, and the thickness of the plate applied, and these decorative nickel plates of various degrees of sub-microscopic and microscopic satin texture and luster when over-laid with a final thin chromium plate provide exceptionally outstanding corrosion protection to the underlying metal.

In our co-pending application Serial 45,287, filed July 26, 1960, we disclosed that barium chromate fine powder in high concentrations stopped the nickel plating. We have now found that while the high concentrations of an insoluble chromate such as barium chromate cannot be safely used to obtain a satin nickel because of the amount of free chromate ion that forms in the bath from the trace solubility of an Tinsoluble chromate like barium chromate when used at high concentrations causes skipped plate, nevertheless at lower concentrations under about .grams/ liter the skipped plate tendency is negligible and while macroscopic lustrous satin nickel is not safely attainable with barium chromate, nevertheless the high corrosion resistance after the usual 0.01 mil final chromium plate is attained with the lower concentrations. This greatly improved corrosion resistance is very worthwhile in itself and very important for bright and semibright nickel plate that is decoratively chromium plated and exposed to outdoor industrial and marine atmospheres, such as decorative hardware on automobiles and boats.

We have found that the addition to semi-bright and bright nickel electroplating baths of low concentrations of water-insoluble oxygen-containing compounds as fine powder of average particle size less than 5 microns and down to colloidal dimensions of at least one material selected from the group consisting of barium chromate in a concentration of 0.1 to about 5 grams/liter, lead chromate in a concentration of 0.1 to about 20 grams/ liter, phosphates of thorium and lead in concentrations of 0.1 to 50 grams/liter, the oxide-s and hydroxides of nickel and cobalt in concentrations of about 1 to 50 grams/ liter, and the stannates of calcium, strontium, barium, lead, nickel, cobalt, and iron in concentrations of about 0.1 to about 20 grams/liter, will produce exceptionally highly corro sion resistant plate when given the usual final over-lay plate of about 0.01 mil (0.25 microns) of chromium plate.

The average particle diameter (herein sometimes referred to as particle size) of the finely powdered in- 3,268,423 Patented August 23, 1966 soluble materials should not be greater than 5 microns. As some roughness, especially on shelf areas where particies can settle may result from the use of materials of particle size greater than about 5 microns, the use of material of particle size less than 5 microns are preferred and are advantageous, with the most preferred particle size averaging about 0.02 to 0.5 micron as determined with the electron microscope. Some agglomerated particles may have larger particle size than 5 microns but with agitation in the nickel bath the larger agglomerates may be reduced to 5 microns and under. Agitation is usually necessary to keep the [fine powder suspended in the baths during plating. Air agitation of the baths can be used. With concentrations of 0.5 to 5 grams/liter of the finest powders, as low as about 0.01% to 0.05% by weight is present in the nickel plate. Microscopic examination of the surface of the plates shows an extremely uniform finely pitted surface which consists of micro-inclusions and micro pits. Calculations and microscopic examinations indicate that the number of micro-inclusions and micro-pits per sq. cm. of surface is of the order of 10 It has been found that in plating articles with recessed areas and with shelf areas using particles of the preferred particle size, no objectionable roughness is obtained on the areas on which settling can occur, though in some cases with the larger particle sizes, the shelf areas may be duller than the rest of the article, though this is usually negligible with short plating times such as 30 seconds to about 5 minutes.

The fine bath-insoluble powders plate out as uniform dispersions in the semi-bright and brig-ht nicked plate and thereby cause sub-microscopic (with the finest particles of 0.01 to about 0.05 micron size) to microscopic-inclusions and sub-microscopic to microscopic-pitting in the surface of the nickel plate. That is, at any given instant the surface of the semi-bright or bright nickel plate has distributed over its surface multitudin-ous fine particles in various stages of being imbedded in the surface and causing sub-microscopic and microscopic pitting, and with the thinnest plates (flashes or strikes) the pitting is mostly sub-microscopic becoming more microscopically visible with thicker plating. When the usual chromium plate of about 0.01 mil (0.25 micron) thickness is applied to these sub-microscopic to microscopic textured surfiaces a very fine favorable porosity pattern is developed in the chromium plate which is the key to the extraordinary corrosion protection afforded to the underlying metal by this composite nickel-chromium plate. With the very fine porosity pattern that is developed in the thin chromium plate there is obtained in corrosive atmospheres the very favorable condition of tiny cathode areas, the chromium surrounding the multitudinous tiny anodes (the Subnnicroscopic and microscopic pits) which results in very weak corrosion currents with very low anodic current densities in the corrosive environments. Thus, the penetration of the corrosion pits toward the underlying basis metal is very greatly diminished. There is also some evidence of extremely fine stress-cracking of the chromium around the micro-inclusions which is also favorable to forming micro-cathodes and anodes. There is also the possibility that with the cromium plate where most of the micro inclusions and micro-pits are not completely plated over, that the chromium in the micro-pits may have some chromium chromate inhibitor formed in the micro-pits which would, besides the poorly conducting particles, also be favorable to minimize the start of anodic attack. However, from the results obtained under prolonged severe corrosion exposure it appears that it is the tiny cathode areas (chromium) surrounding the multitudinous tiny anode (nickel) areas that are responsible for the greatly improved corrosion protection affiorded by the thin chrornium plated sub-microscopic to microscopic-textured finegrained nickel deposits of this invention.

These textured nickel deposits give the best appearance and corrosion protection results when plated on top of semi-bright sulfur-free nickel or bright nickel deposits. It is best and also simpler to use the regular semi-bright or bright nickel plating baths for most of the plate and to use the minimum of the textured nickel plate required to obtain the desired appearance and corrosion resistance, because the textured nickel plating bath requires added control due to the presence of the dispersed particles, and also because the best corrosion protection results are obtained in this way.

The mechanism by which the particles plate out is not clear, the adsorption of hydrogen ions and nickel ions by the particles would give the particles a positive charge and in this way they would tend to plate out. Also, under the powerful reducing conditions at the nickel cathode it might be possible that particles which are not semi-conductors could become semi-conductors by partial reduction. Nevertheless, independent of the mechanism of the plating out of these particles into the nickel plate, it is amazing how readily these particles plate out in a surprisingly uniform manner in semi bright and bright nickel plates. The plating out of the fine particles starts immediately and in the bright nickel plating baths there is evidence that the first layers of nickel plated out actually contain somewhat higher concentrations of the fine powder than the subsequent ones. Also, the first layers may cause more microstress cracking in the thin chromium than the subsequent layers.

Before technical grade powders are used commercially they should always be checked first in small scale tests such as in 1-4 liter baths before being added to large baths because certain harmful impurities may be present such as excess soluble chromate which would cause poor adhesion or skipped plate even with low concentrations of 1 or 2 grams per liter or too coarse particles may be present which would cause rough plate, especially on shelf areas, but otherwise technical grade fine powders normally produce results similar to those obtained from the use of high purity grades of the same particle size and structure. C'hromate powders because of their oxidizing character should not be washed with easily reduced solvents such as alcohol. In general, bright or semi-bright nickel plating baths of the Watts, sulfate, high chloride, sulfam'ate or fluoborate type, or mixtures, can be used. While boric acid is the buffer usually used, other buffers, such as formates, acetates, succinates or citrates may be also employed.

The temperature of the barths can be from room temperature to at least 70 C., though in general a temperature of about 55 C. to about 65 C. is preferred.

The best addition agent or brighteners to achieve the semi-bright and bright nickel plating conditions necessary to obtain the lustrous textured nickel after the addition to the bath of the aforementioned powders are the following: the sulfur-containing brighteners including aromatic and unsaturated aliphatic sulfonic acids, sul-fonamides and sulfonimides, such as the benzeneor naphthalene-sulfonic acids, p-toluene sultonarnide, ben zene sulfonamide, o-benzoyl sulfimide, vinyl sulfonic acid, allyl sulfonic acid, 2-hutyne l, 4-disulfonic acid, sulfobenzaldehyde, etc., the addition agents which produce semi-bright sulfur-free nickel plate such as formaldehyde, chloral hydrate, bromal hydrate, coumarin, butyne diol, adducts of butyne diol, used alone or in combination; combinations of unsaturated addition agents containing unsaturated linkages such as 0:0 C=N, 05c, (JEN with organic sulfur-containing brighteners, organic sulfoncompounds, and combinations of the latter with small concentrations of amines, such as quinaldine, polya-mines or unsaturated amines such as N-al-lyl isoquinolinium bromide, or other quaternaries of pyridines or quinolines or isoquinolines. The leveling of the bright nickel plating baths is not decreased by the presence of the finely powdered additives.

Cobalt and iron can be present in the nickel bath as cobalt or ferrous sulfates, chlorides, bromides, sulfamates or fluoborates in concentrations as high as at least grams/liter, yielding nickel alloy plates containing concentrations of cobalt and/ or iron up to or more and it is to be understood that, except when the context requires otherwise, the expression nickel plate as used herein covers such nickel alloy plates.

Surface active agents may be present in the baths, but are not usually necessary in the air agitated baths.

The maximum increase in lustrous sheen is obtained when the fine powders are used in the agitated full brig-ht nickel plating baths such as the air-agitated bright nickel plating baths possessing good leveling properties. Less luster is obtained when the nickel baths contain only a carrier type brightener such as a benzene or naphthalene sulfonic acid, p-toluene sulfonarnide, benzene sulfonamide or o-benzoyl sulfimide. In the latter cases the luster is flatter. This is also true when the semi-bright sulfurfrce type of addition agent such as formaldehyde, coumarin, chloral hydrate or bromal, is used solely with the fine powders, and with these semi-bright addition agents it is usually best to use the ultra-fine particle size powders of less than about 0.5 micron particle size.

The sub-microscopic and microscopic satin-textured nickel plate accepts chromium plate like regular nickel plate, and in general only the usual thicknesses of final chromium layer need be used, that is 0.25 microns though thicknesses of 2.5 or 5 microns may be used. Besides, the decorative nickel finish as such, or with the usual final chromium finish, the microscopic to macroscopic satin textured nickel plate can be given a rhodium, silver, tin, brass, bronze, copper, gold, or tin-nickel 35) alloy or other final thin coating. Thin wax or soluble wax, films or clear lacquers greatly decrease finger marking of the final coatings, such as nickel, bronze, silver or brass coatings. Chromium, rhodium, and tin-nickel alloy plate do not need these organic coatings.

Below are given examples of baths which give excellent corrosion protection results when microscopic textured bright nickel plate obtained with these baths is given the usual 0.01 mil thick final chromium plate. It is best to use the plates from these baths as thin plates on top of regular bright nickel or regular semibright nickel. Using the CASS or Corrodkote accelerated corrosion tests, many cycles are passed with only 0.6 mil of regular bright nickel or semi bright nickel covered with a thin plate of only about 0.01 to 0.1 mil from baths of Examples 1, II and III and with a final chromium plate of 0.01 mil. In comparison with regular bright nickel alone of the same total plate thickness and with the same final chromium thickness not one cycle is passed.

Excellent corrosion protection results after the usual final thin chromium plate are obtained by using mixtures of fine powders, for example, barium chromate at 0.05 to 1 gram/liter with 1 to 5 grams/liter of lead phosphate or thorium phosphate or lead chromate. Thorium phosphate used in total concentrations of 0.1 to about 20 grams/liter in conjunction with about 1 to 50 grams/liter of very fine silica powder also gives ex cellent results. With the very fine silica dispersed in the bright nickel plating baths at about 10 to 50 grams/ liter and thorium phosphate, or lead phosphate, or lead chromate fine powder dispersed in a concentration of 0.1 to 5 grams/liter, excellent corrosion protection results even in deep recesses are obtained when only about 0.01 to 0.1 mil of the sub-microscopic to microscopic textured bright nickel plate is plated upon regular bright nickel or semi-bright nickel of total nickel thickness of only about 0.4 mil, and with a final 0.01 mil of chromium on top of the textured bright nickel plate.

Example I Grams/liter Lead chromate 0.02-3 microns av. particle particle size 0.05-5 NiSO .6H O 200-300 NiCl .6H O 40-80 H BO 40 o-Benzoyl sulfimide 1-3 p-Toluene sulfonamide 1-2 Allyl sulfonic acid 1-2 N-allyl quinaldinium bromide 0003-001 pH=4.5-5.5, temp. 50-70 C.

Air agitation.

Example II Lead phosphate and/or thorium phosphate 0.02-5 micron av. particle size 0.1- NiSO .6H O 200-300 NiCl .6H O 40-80 H BO 40 Benzene sulfonamide 1-3 Allyl sulfonic acid 1-3 2-butynoxy-1, 4-diethane sulfionic acid 0.050.2

pH=3.( -5.2, temp. 5065 C.

Air agitation or mechanical agitation.

Example III Thorium phosphate 0.02-3 micron av. particle size 0.1-5 Ultra-fine silica powder 0.02-0.05 micron ultimate particle size (Quso) 1-50 NiSO .6H O 50-200 NiCl .6H O 200-100 H BO 40 o-Benzoyl sulfimide 2-4 p-Toluene sulfonamide 1-2 Allyl sulfonic acid 1-3 2-butynoxy-l, 4-diethane sulfonic acid 0.05-0.2

pH=3.0-5.2, temp. 5070 C. Air agitation or mechanical agitation.

It is to be understood that other inorganic bath compositions can be used and other nickel brighteners, but it is preferred to use at least one organic sulfon-compound as one of the brighteners.

When the ultra-fine particles of about 0.01 to 0.05 micron size particles are used in the semi-bright and bright nickel electroplating bath, it is diflicult to see the included particles in cross-section of the nickel plate even at the highest magnification of the light microscope. However, on the surface of the plate using strong light it is possible to just see the microscopic pitting effect of these submicroscopic particles. The thinner the plate that is deposited on a bright surface, the more difficult is it to distinguish any difference between the appearance of the textured deposit and the bright plate obtained without the particles present.

The nickel brighteners that produce very high leveling and brilliance as, for eXample, those given in US. 2,647,866 (Aug. 4, 1953) and U.S. 2,800,440 and 2,800,- 442 (July 23, 1957) will produce the highest brilliance with the powders dispersed in these bright nickel baths.

To achieve the highest possible corrosion protection results with the textured decorative nickel plate of this invention on complex shaped articles such as many zinc die-cast articles, for example, rear view mirror holders, intricate light housings, steel bumpers, hub caps, and grilles, it is best to use duplex or dual nickel underneath the textured nickel deposit. Thus, the total nickel deposit would consist of semi-bright sulfur-free nickel followed by regular bright nickel followed by a thin textured nickel deposit of this invention. The latter being used as thin plate (0.01 to about 0.1 mil) if the highest brilliance is desired. If between the semi-bright sulfurfree nickel and the regular bright nickel, a thin plate (0.02 to 0.1 mil) of higher sulfur content (as nickel sulfide) nickel plate (0.1 to 0.2% sulfur) than the bright nickel plate (0.05 to 0.08% sulfur) is used, then with this tri-nickel plate with a final textured nickel plate of this invention before the top thin chromium plate, even the thinnest total nickel plate (0.3 mil) in deeply recessed articles stands up extremely well in very corrosive atmospheres. When ductile copper plate is used under nickel plate that has a final coating of the textured decorative nickel plate of this invention, then the copper plate also helps in the total corrosion resistance unlike the case when copper is used as a substitute for part of the bright nickel thickness in deposits of copper-bright nickel and the usual 0.01 mil thick final chromium. It is believed that this beneficial effect of copper is also due to the tiny cathode areas developed in the final thin chromium plate, which in turn is due to the fine favorable porosity pattern developed in the thin final chromium plate as a result of its being deposited over a decorative nickel surface containing multitudinous sub-micro to micro-inclusions and sub-micro to micro-pits of the order of 10 per sq. cm.

The nickel hydroxide powder can be formed in the bath as a colloid by adding alkalis, such as sodium hydr-oxide or barium hydroxide, to the nickel bath. In general, the preferred fine powder is thorium phosphate, since it is the least critical to use from the standpoint of the pH of the bath (pH=3-6) and the concentration of the powder. With most of the other powders it is best to use the higher pH values of the nickel baths (pH values around 5.5 to 6.0). The preferred phosphates of thorium and of lead are the orthophosphates, that is, the preferred phosphates are the most Water-insoluble ones. The preferred oxides of nickel and cobalt are the divalent ones, and dead burnt ones.

The oxides and hydroxides of nickel and cobalt seem to acquire a positive charge by adsorption of hydrogen ions and nickel ions in the nickel baths at pH values of about 5.2-6.0 and this seems to occur with the other types of precipitated powders such as the water-insoluble stannates, for example, calcium, barium and strontium stannates, as well as lead and barium chromates, and thorium and lead phosphates. The acquiring of a positive charge by adsorption appears to the main mechanism by which these particles plate out into the nickel cathode from the semi-bright and bright nickel plating baths.

The best of the water-insoluble stannate powders to use are those of strontium and barium, with calcium, thorium, cerium next, and lead, antimony, bismuth, magnesium, cobalt, nickel and iron next. In some cases higher than 20 grams/liter concentration can be used, but for lead, antimony, and bismuth it is preferred to use the lower concentration values. The hypophosphate of thorium is nearly as insoluble as the thorium orthophosphate and gives very good results in concentrations of 0.1 to about 50 grams/liter. Furthermore, it was found that the water-insoluble oxalates of thorium and lead give very similar results to the phosphates of thorium and lead in the concentrations of 0.1 to about 20 grams/liter, with the thorium oxalate giving superior results both from the standpoint of uniform decorative appearance and corrosion protection results after the final thin chromium plate. In general, lead oxalate just like lead stannate should be used at the lower concentration values of 0.1 to about 3 grams/liter for best results. At the higher concentration values these lead salts introduce too much free lead ions in the baths which tend to darken the low current density plate, and to decrease the adhesion of the plate. It is also preferred for this reason to use the lead salts in the sulfate type of bright nickel, e.g., bright nickel baths based on the Watts bath inorganic formulation.

The best powders of this invention from all standpoints, that is, least critical in controland operation of the baths, corrosion results after the final thin chromium plate is applied, and best decorative appearance are, thorium phosphates and oxalate, lead chromate, lead phosphate, and strontium and barium stannates. These powders can be used at bath pH values of 3-6. With strontium and barium stannate fine powders dispersed in the bright nickel baths using Watts type formulations the 10 to 20 gram/liter concentrations of these powders give very excellent corrosion results with 30 seconds to 3 minutes plates on top of regular bright or semi-bright nickel plate of only 0.4 mil total nickel thickness and 0.01 mil final chromium thickness.

What is claimed is:

1. A method for electrodepositing a decorative nickel plate which comprises the step of electrolyzing an aqueous acidic solution of at least one nickel salt selected from the group consisting of nickel sulfate, nickel chloride, nickel fluoborate and nickel sulfamate and at least one soluble organic addition agent capable of producing semi-bright to fully bright nickel plate, said solution containing dispersed therein about 0.1 to about 50 grams/ liter of at least one water-insoluble material selected from the group consisting of the phosphates of lead, about 0.1 to 20 grams/liter of lead chromate, and lead oxalate, 0.1 to about 5 grams/liter of barium chromate, 1 to about '50 grams/liter of the oxides and hydroxides of cobalt and nickel, and 0.1 to about 20 grams/liter of the stannates of calcium, strontium, barium, magnesium, lead, nickel, cobalt, iron, antimony and bismuth, said material being in the form of a particulate dispersion the particles of which having an ultimate particle size of less than about 5 microns average diameter, and thereafter plating on said electro-deposited layer an overlayer of a metal selected from the group consisting of chromium, rhodium, silver, tin, brass, bronze, copper, gold and an alloy consisting of 65 tin and 35 nickel, said overlayer having a thickness less than about 5 microns.

2. A method in accordance with claim 1 wherein said fine powder is lead chromate.

3. A method in accordance with claim -1 wherein said fine powder is lead phosphate.

4. A method in accordance with claim 1 wherein said fine powder is strontium stannate.

5. A method in accordance with claim 1 wherein the metal of said overlayer is chromium.

6. A method in accordance with claim 1 wherein said nickel plate is electrodeposited directly on an electrodeposit consisting essentially of nickel.

7. A method in accordance with claim 1 wherein said dissolved organic nickel brightener is selected from the group consisting of aromatic and unsaturated sulfonic acids, sulfonamides and sulfonimides.

References Cited by the Examiner UNITED STATES'PATENTS 2,739,085 3/1956 McBride 204181 X 2,771,409 11/ 1956 Cross 204-40 X 3,057,048 10/ 1962 Hirakis 20449 X 3,061,525 10/1962 Grazen 204-9 3,090,733 5/ 1963 Brown 20440 3,132,928 5/ 1964 Crooks et al 204-40 X JOHN H. MACK, Primary Examiner.

ALLEN B. CURTIS, Examiner.

G. KAPLAN, Assistant Examiner. 

1. A METHOD FOR ELECTRODEPOSITING A DECORATIVE NICKEL PLATE WHICH COMPRISES THE STEP OF ELECTROLYZING AN AQUEOUS ACIDIC SOLUTION OF AT LEAST ONE NICKEL SALT SELECTED FROM THE GROUP CONSISTING OF NICKEL SULFATE, NICKEL CHLORIDE, NICKEL FLUOBORATE AND NICKEL SULFAMATE AND AT LEAST ONE SOLUBLE ORGANIC ADDITION AGENT CAPABLE OF PRODUCING SEMI-BRIGHT TO FULL BRIGHT NICKEL PLATE, SAID SOLUTION CONTANING DISPERSED THEREIN ABOUT 0.1 TO ABOUT 50 GRAMS/ LITER OF AT LEAST ONE WATER-INSOLUBLE MATERIAL SELECTED FROM THE GROUP CONSISTING OF THE PHOSPHATES OF LEAD, ABOUT 0.1 TO 20 GRAMS/LITER OF LEAD CHROMATE, AND LEAD OXALATE, 0.1 TO ABOUT 5 GRAMS/LITER OF BARIUM CHROMATE, 1 TO ABOUT 50 GRAMS/LITER OF THE OXIDES AND HYDROXIDES OF COBALT AND NICKEL, AND 0.2 TO ABOUT 20 GRAMS/LITER OF THE STANNATES OF CALCIUM, STRONTIUM, BARIUM, MAGNESIUM, LEAD, NICKEL, COBALT, IRON, ANTIMONY AND BISMUTH, SAID MATERIAL BEING IN THE FORM OF A PARTICULATE DISPERSION THE PARTICLES OF WHICH HAVING AN ULTIMATE PARTICLE SIZE OF LESS THAN ABOUT 5 MICRONS AVERAGE DIAMETER, AND THEREAFTER PLATING ON SAID ELECTRO-DEPOSITED LAYER AN OVERLAYER OF A METAL SELECTED FRM THE GROUP CONSISTING OF CHROMIUM, RHODIUM, SILVER, TIN, BRASS, BRONZE, COPPER, GOLD AND AN ALLOY CONSISTING OF 65 TIN AND 35 NICKEL, SAID OVERLAYER HAVING A THICKNESS LESS THAN ABOUT 5 MICRONS. 